Laser having means for defogging the optical cavity thereof

ABSTRACT

In a laser, having a lasing medium and having an optical resonator and wherein there is a space or cavity between the lasing medium and the mirrors forming the optical resonator, means is provided for removing contamination particles, such as dust and fog from the cavity region in the path that the reflected coherent light beam passes. This is accomplished by providing a radioactive source for electrostatically charging the contamination particles and then removing the charged particles from the path of the light beam by, for example, an electrical field.

United States Patent Manoukian Jan. 1, 1974 LASER HAVING MEANS FORDEEOGGING THE OPTICAL CAVITY THEREOF Nubar S; Manoukian, San Jose,Calif.

Coherent Radiation, Palo Alto, Calif.

Filed: Feb. 4, 1972 Appl. No: 223,602

Inventor:

Assignee:

US. Cl 331/945, 55/102, 250/106 R Int. Cl...; H015 3/22 Field of Search331/945; 250/42,

References 1 Cited UNITED STATES PATENTS 8/1963 Scott-Maxwell 250/106 R2/1971 Rigden et a1 331/945 11/1971 Kaiser et a1 33l/94.5

FOREIGN PATENTS OR APPLICATIONS 722,933 2/1955 Great Britain 55/102Primary Examiner-Ronald L. Wibert Assistant Examiner-R. J. WebsterAtt0meyKarl A. Limbach et a1.

[ 5 7 ABSTRACT In a laser, having a lasing medium and having an opticalresonator and wherein there is a space or cavity between the lasingmedium and the mirrors forming the optical resonator, means is providedfor removing contamination particles, such as dust and fog from thecavity region in the path that the reflected coherent light beam passes.This is accomplished by providing a radioactive source forelectrostatically charging the contamination particles and then removingthe charged particles from thepath of the light beam by, for example, anelectrical field.

15 Claims, 3 Drawing Figures LASER HAVING MEANS FOR DEFOGGING THEOPTICAL CAVITY THEREOF BACKGROUND OF INVENTION Many lasers include aspace or cavity located between the lasing medium and the mirrorsforming the optical resonatoir of the laser. For example, in a gaseouslaser, a gas, the active medium, is hermetically sealed within adischarge tube and is then placed between a pair of mirrors forming theoptical resonator of the laser. In operation, the gas or gasses withinthe discharge tube are energized to create the condition commonlyreferred to as inverted population of energy levels. As the atoms, ionsor molecules, as the case may be, decay to lower energy levels theresulting light energy emitted is reflected by the optical resonatorwhich is axially aligned with the discharge tube in which the light isamplified.

In the area between the lasing medium and the mirrors forming theoptical resonator structure, it frequently occurs that dust, fog andother contaminating particles accumulate. Thus, in the example of agaseous ion laser, these contamination particles build up between thedischarge tube and the mirrors of the resonator. Since many gaseous ionlasers include a pair of windows, typically at Brewsters angle to thedirection of the reflected laser beam, which form a part of thedischarge tube, the particular area of contamination buildup is in thecavity region between the Brewster windows and the mirrors of theoptical resonator.

The existence of the contaminating particles results in decreasedtransmission of light between the mirrors of the optical resonator. Thisresults in reduced power and can eventually result in complete failureof the laser to emit any light.

It has been found that there are two major sources of contaminatingparticles in the cavity region. The first is water vapor or fog. It isbelieved that this fog oc curs as a result of condensation of watervapor from the atmosphere around ionized gaseous molecules which areformed due to ultra-violet light generated by the laser itself. Theproblem of fogging appears to be more prevalent where the ambientatmospheric conditions are characterized by high humidity. Anothersource of contamination is the dust which circulates throughout theatmosphere.

Many attempts have been made to eliminate the contaminating particlesfrom the cavity regions of the laser. One approach is to hermeticallyseal the cavity region. However, this is not satisfactory because minutevibrations are then transmitted from the discharge tube to the mirrorsforming the optical resonator. This results in noise in the output lightbeam.

Another approach is to continuously provide the area with a clean drystream of air. However, this results in the formation of dirt on thesurfaces of the mirrors and of the resonator and the Brewster windows,where provided. This causes the light reflected within the laser to besignificantly reduced. The same undesirable results have been found tooccur even in a closed type recirculation system.

In one application a desiccant was provided in the cavity region.However, this did not prove to be successful because there wasinsufficient convection currents to adequately cleanse the area.

Other attempted solutions have included the use of a heater to dissipatethe fog. This is not satisfactory because the high temperaturesintroduced within the laser cause misalignment of the mirrors and alsodo not solve the dust problem. Also, an ultra-violet filter wasintroduced within the optical resonator. This works satisfactorily toget rid of the fog but has an adverse effect on the output mode and alsodoes not eliminate the dust problem.

SUMMARY OF THE INVENTION In accordance with the invention, means fordispersing contaminating particles suspended in the atmosphere in thecavity region of the laser located generally between the mirrors formingthe optical resonator and the active lasing material is provided. Inparticular, the dispersing means moves the contaminating particles fromat least that portion of the cavity through which the light beam,reflected between the mirrors of the optical resonator, passes.

The contaminating particles are electrostatically charged by means ofradioactive emissions from a radioactive source located in the vicinityof the cavity. The charged contamination particles are then removed fromthe area through which the beam passes by, for example, an electricalfield which attracts the charged particles to the pole having theopposite polarity to that of each of the charged particles.

In the preferred embodiment of the invention the radioactive material isenclosed within a contamination or dust shield which connects andencloses a portion of the area between the mirrors of the opticalresonator and the ends of the capsule enclosing the active lasermaterial. In the example of a gaseous laser, the dust shield includingthe radioactive material extends from the Brewster-windows of thedischarge tube to the respective mirrors of the optical resonator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof one laser embodiment employing the present invention.

FIG. 2 is a detailed elevational drawing, partially in section, of apart of an actual laser employing the present invention.

FIG. 3 is a cross-sectional view of that part of the laser shown in FIG.2 in the direction indicated by the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Illustrated schematically inFIG. 1 is a laser 10. For purposes of describing the invention, thelaser depicted is a gaseous ion laser. Thus, it includes a high currentgas discharge tube or chamber 12 including an anode l4, cathode 16 and abore structure 18, located centrally thereof and defining an arcdischarge path 20 as shown. The are discharge is established by applyinga voltage from a power supply 21 across the anode 14 and cathode 16. Thegas chamber 12 is sealed and contains a gas such as argon or krypton orother suitable gas. The gas is the active lasing medium.

The anode and cathode are concentric and permit passing of an opticalbeam created within the'gas chamher to pass through windows 22 and 24arranged at Brewsters angle to the path of the internally reflectedcoherent light beam. I

The arc discharge causes the gases within the gas chamber 12 to beionized and excited to high energy levels and as these ions decay tolower energy states radiation is emitted in a manner well known to thoseskilled in the art. By providing an optical resonator structure 25 whichincludes a first mirror 26 which is substantially totally reflecting anda second mirror 28 which is approximtely 97 98 percent reflecting, theresulting radiation is amplified in a manner also well known to thoseskilled in the laser art and an output is provided through the mirror28. For a more detailed description of an optical resonator structure,reference is made to co-pending US. Pat. application, Ser. No. 842,956by Wayne SQMefferd and James L. Hobart, entitled Laser AlignmentApparatus, filed July 18, 1969, and assigned to the same assignee as thepresent invention.

Since the operation of the laser is at very high temperatures, in thevicinity of l,OC., a cooling system 30 is provided which includes acooling jacket 32 surrounding the gas chamber. Water enters in throughone tube 34 into the cooling jacket 32 and returns to the cooling systemthrough tube 36. A solenoidal electromagnet 38 surrounds the gas chamber12 and provides an axial magnetic field. The function of the field is toincrease the power output by confining and thereby increasing the iondensity without lowering the electron energy enough to degrade the laserexcitation. The magnetic field also decreases the formative time lag inarc initiation by the same mechanism. The solenoid 38 is energized fromthe power supply 21.

The entire laser assembly, including the resonator structure 25 and thedischarge structure 18 is enclosed by a jacket or cover 40. v

Each of the regions between the Brewster windows 22 and 24 and themirrors 26 and 28 respectively of the optical resonator structure 25define a space or cavity 42. As previously explained, this area isfilled with air and often becomes contaminated with particles such asdust and water vapor or fog. The latter is particularly prevalent wherethe ambient air environment has a high relative humidity. Contaminationor dust shields 44 enclose and connect the Brewster windows 22 and 24with the mirrors 26 and 28 of the optical resonator structure. Asexplained above, these shields are not hermetically connected betweenthe discharge tube and the mirrors because vibrations generated withinthe discharge tube would then be transmitted to the mirrors and noise inthe output beam of the laser would be created. Thus, although the dustshields tend to reduce the amount of flow of atmosphere in the areaenclosed by the shield, it does not completely prevent the circulationof air, and hence dust and fog, in the region through which the laserbeam passes.

Reference is now made to FIGS. 2 and 3 which ilustrate the details of apart of an actual laser which utilizes the subject invention. Forpurposes of illustration, only one of the two cavity portions of thelaser is depicted therein. The dust shield 44 includes a tubular glasssection 46 and an ionizing chamber 48. The latter is secured at one endto the mirror support assembly 28 which forms one part of the opticalresonator 25. The glass tube 46 is secured to the ionizing chamber 48 byany suitable means such as the threaded arrangement 50. Suitable sealingmeans such as an O-ring 52 is provided to reduce the circulation of airtherethrough.

The other end of the glass tube 46 is secured to one end of thedischarge tube 54. This part of the discharge tube 54 includes theBrewster window 24. Again, any

suitable means may be employed to attach the glass tube 46 to the end 54of the discharge tube.

In accordance with the invention, a source of radioactive radiation 56,enclosed in a capsule 58, is mounted within the ionizing chamber 48 bymeans of an adhesive such as epoxy at several points 62 to the insidewalls of the ionizing chamber 48. Preferably, the ionizing chamber 48 ismade of aluminum.

The radioactive element 56 is covered at the open side of the capsule 58by a protective grid 64 which is held in place by a clip 60 which formspart of the capsule 58. The purpose of this grid is to prevent the solidradioactive material 56 from being touched or handled. To furtherprotect the user, the radioactive material is sealed between a silverbase and a layer of gold (not shown).

As previously explained, dust particles and fog within the cavity 42 areelectrostatically charged by dense isotope radiation from the radiationsource 56. To remove the charged particles from the region 42 throughwhich the laser beam passes, two electrodes 66 and 68 extend axiallyalong the length of the chamber 48 and an electrical field is createdbetween the electrodes in a direction transverse to the path of thelaser beam. The charged dust and fog particles are swept away by thefield to the electrode of opposite polarity where they collect.

It has been found that a voltage differential of 250 volts is sufficientto sweep away or disperse the charged particles. The electrodes areelectrically insulated from the walls of the chamber 48 by means ofteflon insulator inserts 70.

Polonium-2l0 is a suitable source of radiation. Polonium is harmlessunless ingested or inhaled. Alpha radiation emitted from the polonium islow, about 40 microcuries. Although alpha rays travel rapidly throughair, they lack the power to penetrate the skin. The effective life ofthe radiation is about two years. In that time, enough of theradioactivity will have decayed so that the radioactive element must bereplaced.

It has been found that any contaminating particles present in the cavity42 will be dispersed within a period of time of about 5 minutes to 12hours, depending upon the initial amount of the contaminating particles.

Although the invention has been shown and described in a gaseous ionlaser, the present invention is equally applicable to any laser havingan active medium and an optical resonator and wherein there is an airspace located between the laser medium and the mirrors forming theoptical resonator.

What we claim is:

1. Laser apparatus comprising a. a sealed discharge chamber containingat least one b. means for energizing said at least one gas to at leastone excited state, an excited state being characterized by an invertedpopulation;

0. broad band optical resonating means comprising a first substantiallytotally reflecting mirror located axially of one end of said dischargechamber and a second partially reflecting mirror located axially of theother end of said discharge chamber, said partially reflecting mirrorthereby providing an optical output resulting from the depopulation ofsaid at least one gas; and

(1. means for dispersing particles suspended in the cavity region of thelaser located generally between each of the mirrors forming theresonating means and the respective ends of the discharge chamber andthrough which the reflected coherent light passes, said means comprisingi. a source of radioactive material providing radioactive emissionswithin said cavity region to electrically charge said particles, and

ii. means for attracting said charged particles away from the cavityregion through which the light reflected within said optical resonatorpasses.

2. Laser apparatus as in claim 1 wherein said attracting means comprisesan electric field extending substantially transversely across the pathof the reflected light for at least a portion thereof.

3. Laser apparatus as in claim 1 wherein said discharge tube includes apair of windows at opposite ends thereof wherein light passing withinsaid resonator enters and exits through said windows.

4. Laser apparatus as in claim 3 including a shield extending axiallyfrom each of said windows to the respective mirrors forming saidresonator cavity, said shield being sealed to reduce the freecirculation of air therethrough.

5. Laser apparatus as in claim 4 wherein said radioactive material issecured to said shield.

6. Laser apparatus as in claim 5 wherein said attracting means comprisesan electric field.

7. Laser apparatus as in claim 6 wherein said electric field is createdby applying a voltage across a pair of electrodes secured to saidshield.

8. Laser apparatus as in claim 1 wherein said radioactive materialcomprises radioactive p'olonium.

9. Laser apparatus as in claim 3 wherein said windows are situated atBrewsters angle relative to the path of the reflected light.

10. In a laser having a lasing medium, means for energizing said lasingmedium to an excited condition characterized by an inverted population,optical resonating means comprising a pair of axially aligned mirrors,one of which is only partially reflective and wherein an ambientatmosphere filled cavity exists between at least one of said mirrors andsaid lasing medium, and wherein the improvement comprises:

a. means for dispersing contaminating particles suspended in said cavityat least in the region through which coherent light reflected betweensaid mirrors passes, said dispersing means comprising b. a source ofradioactive material providing radioactive emissions within said cavityto charge said contaminating particles therein, and Y 0. means forattracting said charged particles away from the cavity region throughwhich said reflected light passes.

11. Laser apparatus as in claim 10 wherein said attracting meanscomprises an electric field extending substantially transversely acrossthe path of the reflected light for at least a portion thereof.

12. Laser apparatus as in claim 10 including a shield enclosing at leasta part of said cavity, said shield extending from said mirror of saidresonator to said lasing medium, and wherein said shield is sealed toreduce the free circulation of air therethrough.

13. Laser apparatus as in claim 12 wherein said radioactive material issecured to said shield.

14. Laser apparatus as in claim 13 wherein said attracting meanscomprises an electric field created by applying a voltage across a pairof electrodes secured to said shield.

15. Laser apparatus as in claim 14 wherein said radioactive materialcomprises radioactive polonium.

1. Laser apparatus comprising a. a sealed discharge chamber containingat least one gas; b. means for energizing said at least one gas to atleast one excited state, an excited state being characterized by aninverted population; c. broad band optical resonating means comprising afirst substantially totally reflecting mirror located axially of one endof said discharge chamber and a second partially reflecting mirrorlocated axially of the other end of said discharge chamber, saidpartially reflecting mirror thereby providing an optical outputresulting from the depopulation of said at least one gas; and d. meansfor dispersing particles suspended in the cavity region of the laserlocated generally between each of the mirrors forming the resonatingmeans and the respective ends of the discharge chamber and through whichthe reflected coherent light passes, said means comprising i. a sourceof radioactive material providing radioactive emissions within saidcavity region to electrically charge said particles, and ii. means forattracting said charged particles away from the cavity region throughwhich the light reflected within said optical resonator passes.
 2. Laserapparatus as in claim 1 wherein said attracting means comprises anelectric field extending substantially transversely across the path ofthe reflected light for at least a portion thereof.
 3. Laser apparatusas in claim 1 wherein said discharge tube includes a pair of windows atopposite ends thereof wherein light passing within said resonator entersand exits through said windows.
 4. Laser apparatus as in claim 3including a shield extending axially from each of said windows to therespective mirrors forming said resonator cavity, said shield beingsealed to reduce the free circulation of air therethrough.
 5. Laserapparatus as in claim 4 wherein said radioactive material is secured tosaid shield.
 6. Laser apparatus as in claim 5 wherein said attractingmeans comprises an electric field.
 7. Laser apparatus as in claim 6wherein said electric field is created by applying a voltage across apair of electrodes secured to said shield.
 8. Laser apparatus as inclaim 1 wherein said radioactive material comprises radioactivepolOnium.
 9. Laser apparatus as in claim 3 wherein said windows aresituated at Brewster''s angle relative to the path of the reflectedlight.
 10. In a laser having a lasing medium, means for energizing saidlasing medium to an excited condition characterized by an invertedpopulation, optical resonating means comprising a pair of axiallyaligned mirrors, one of which is only partially reflective and whereinan ambient atmosphere filled cavity exists between at least one of saidmirrors and said lasing medium, and wherein the improvement comprises:a. means for dispersing contaminating particles suspended in said cavityat least in the region through which coherent light reflected betweensaid mirrors passes, said dispersing means comprising b. a source ofradioactive material providing radioactive emissions within said cavityto charge said contaminating particles therein, and c. means forattracting said charged particles away from the cavity region throughwhich said reflected light passes.
 11. Laser apparatus as in claim 10wherein said attracting means comprises an electric field extendingsubstantially transversely across the path of the reflected light for atleast a portion thereof.
 12. Laser apparatus as in claim 10 including ashield enclosing at least a part of said cavity, said shield extendingfrom said mirror of said resonator to said lasing medium, and whereinsaid shield is sealed to reduce the free circulation of airtherethrough.
 13. Laser apparatus as in claim 12 wherein saidradioactive material is secured to said shield.
 14. Laser apparatus asin claim 13 wherein said attracting means comprises an electric fieldcreated by applying a voltage across a pair of electrodes secured tosaid shield.
 15. Laser apparatus as in claim 14 wherein said radioactivematerial comprises radioactive polonium.